1. Field of the Invention
The present invention relates to an Organic Light Emitting Device (OLED).
2. Description of the Related Art
An OLED is a self-light emitting device which emits light by rejoining electrons with holes in a light emitting layer. An OLED is fabricated as a thin film and has a quick reaction rate and low driving voltage compared to a passive device such as a liquid crystal display (LCD), which requires an additional light source. Thus, the OLED is used in wall-mounted television (TV) sets or portable TVs. Further, the OLED includes a pixel circuit unit having sub-pixels of Red (R), Green (G) and Blue (B) colors to realize a full color display.
In more detail, FIG. 1 illustrates a related active matrix-type OLED 100. As shown, the related art active matrix-type OLED 100 includes a pixel circuit unit 110 for displaying an image, and a data driver 120 and a scan driver 130 for supplying electrical signals to the pixel circuit unit 110.
The data driver 120 supplies data signals to R, G and B sub-pixels 170 of the pixel circuit unit 110 through data lines 140. Further, the scan driver 130 supplies scan signals or selection signals to the R, G and B sub-pixels 170 through scan lines 150. A controller (not shown) supplies R, G and B control signals and scan control signals to the data driver 120 and the scan driver 130, respectively, to control the data driver 120 and the scan driver 130.
The pixel circuit unit 110 also includes a plurality of unit pixels 160. As shown, each unit pixel 160 includes R, G and B sub-pixels 170 positioned in a region defined by the crossing of the data lines 140 and the scan lines 150, respectively and each sub-pixel 170 includes a first electrode, a second electrode, and an organic light emitting layer formed between the first and the second electrodes. The color of each sub-pixel is determined by the organic light emitting layer.
In addition, when electrical signals are supplied to the R, G and B sub-pixels 170 through the data lines 140 and the scan lines 150, the R, G and B sub-pixels 170 emit light having a brightness corresponding to the electrical signals to thereby form a predetermined image in the pixel circuit unit 110.
Turning next to FIG. 2, which is a plane view of a unit pixel showing how sub-pixels are arranged in the related art OLED. FIG. 1 will also be referred to in this description. As shown in FIGS. 1 and 2, each unit pixel 160 includes the R, G and B sub-pixels 170 of a same size in the ratio of 1:1:1 in the horizontal or vertical direction. However, the R, G and B sub-pixels 170 have a different luminous efficiency and life-time expectancy according to materials that form the R, G and B organic light emitting layer. For example, a brightness ratio of the R, G and B sub-pixels is 2.5:5:1 when the white balance follows the National Television System Committee (NTSC), i.e., 0.310 and 0.3130.
Therefore, when the luminous efficiency ratio of the R, G and B sub-pixels 170 is the same as the brightness ratio, the driving current is supplied to each sub-pixel in the same ratio as the brightness ratio. However, the luminous efficiency of the related art R, G and B sub-pixels is different from the brightness ratio, which is used for adjusting the white balance. Thus, when the luminous efficiency ratio of the R, G and B sub-pixels is 1:4:1.5, the ratio of the driving current of R, G and B sub-pixels is approximately 4:2:1.
In addition, the R, G and B sub-pixels are connected to thin film transistors supplied with a driving current. As described above, the driving current of the R, G and B sub-pixels is different, and therefore the performance of the thin film transistor is based on the sub-pixels of the one color that requires the highest driving current among the R, G and B sub-pixels. Further, the more difference between the driving current of the R, G and B sub-pixels is, the more difficult it is to set up a range of an exact R, G and B independent operation. Moreover, even though the same or lower driving current of sub-pixels for one color is required, the life-time expectancy of the sub-pixels for the one color may be shorter than the other colors due to a reliability problem of the sub-pixels themselves.
In addition, because the related art OLED 100 described above has the R, G and B sub-pixels in the ratio of 1:1:1, the driving current applied to each sub-pixel to emit the requested brightness of each color to realize a white balance is determined according to color coordinates and luminous efficiency. Accordingly, the driving current applied to the sub-pixels of a color with a shorter life-time expectancy than those of the other colors cannot be reduced. This disadvantageously affects the life-time expectancy of the pixel circuit unit 110.